In one type of radar system, the transmitter is driven by a CW signal having a frequency that varies linearly with time. The signal is derived from a voltage controlled oscillator (VCO), and the input signal to the VCO is controlled to produce the linear frequency sweep. The return signal from a target is mixed with the transmitted signal to produce a difference signal having a frequency equal to the difference between the instantaneous frequency of the transmitted signal and the return signal. Because the frequency of the transmitted signal changes linearly with time, the frequency of the difference signal is a function of target range.
In the system of the type described above, there are four criteria that must be met to develop an effective radar. The first criteria is that the frequency sweep must be highly linear, in order for the radar to have adequate range resolution. This is a difficult problem, because even the best RF sources have nonlinear tuning characteristics, i.e., the frequency of the VCO output signal is not a linear function of the input voltage. The second criteria is that the frequency sweep must be repeatable, i.e., must have constant starting frequency and slope, so that the radar will have Doppler capability. This requirement restricts the choice of the clock and reference frequencies. The third criteria is that the sweep synthesizer must be one that minimizes cost and size. The fourth criteria in designing an effective FW CW radar is that the linearization of the sweep must be able to adapt immediately to changes in the tuning characteristics of the VCO, which tuning characteristics depend on the terminating impedance at the RF output. This terminating impedance does not remain constant because the antenna must scan, and because polarization switching may occur.
Past efforts to produce a fast, linear frequency sweep have involved both open loop and closed loop designs. To date, open loop techniques have proved nearly impossible to align. If and when they are aligned, they are still sensitive to changes in temperature. Closed loop designs have included phase lock loops in which a signal representing the rate of change of the frequency of the transmitted signal is phase locked to a crystal source. One successful implementation of such a closed loop design is described in U.S. patent application Ser. No. 902,658, filed Sep. 2, 1986, entitled Apparatus and Method for Producing Linear Frequency Sweep. However, the approach described in that application has the disadvantage that it requires three RF sources, three RF mixers, and a delay line.